A "zenith angle" effect results in the erroneous inference of IR brightness
temperatures at pixels that are at relatively high viewing angles relative to
nadir and are inferred to be considerably colder than they are in reality.
Failure to correct for this effect results in discontinuities at the boundaries
of adjacent geostationary satellites as demonstrated in
Figure 1. A relatively crude correction for zenith angle effects on GPI precipitation
estimates was determined by comparing retrieved IR temperature information
from the GOES satellites which have a relatively large region of overlap and
thus allowed the comparison of retrieved IR brightness temperatures from the
same earth locations but at varying viewing angles (Joyce and Arkin
1997). More recent work by Joyce et al. (1999) shows
that the viewing angle effect is dependent on the temperature of the retrieved
pixel temperature, and that a seasonal adjustment is necessary in the extratropical
regions (Figure 2). It is the incorporation of the results of these studies that
enables the production of a nearly seamless map of IR data.